A transformer is a device that transfers electrical energy from a primary circuit to a secondary circuit by magnetic coupling. Typically, a transformer includes one or more windings wrapped around a core. An alternating voltage applied to one winding (a “primary winding”) creates a time-varying magnetic flux in the core, which induces a voltage in the other (“secondary”) winding(s). Varying the relative number of turns of the primary and secondary windings about the core determines the ratio of the input and output voltages of the transformer. For example, a transformer with a turn ratio of 2:1 (primary:secondary) has an input voltage that is two times greater than its output voltage. It is well known in the art to cool high-power transformers using a dielectric fluid, such as a highly-refined mineral oil. The dielectric fluid is stable at high temperatures and has excellent insulating properties for suppressing corona discharge and electric arcing in the transformer. Typically, the transformer includes a tank that is at least partially filled with the dielectric fluid. The dielectric fluid surrounds the transformer core and windings.
Over-current protection devices are widely used to prevent damage to the primary and secondary circuits of transformers. For example, distribution transformers have conventionally been protected from fault currents by high voltage fuses provided on the primary windings. Each fuse includes fuse terminations configured to form an electrical connection between the primary winding and an electrical power source in the primary circuit. A fusible link or element disposed between the fuse terminations is configured to melt, disintegrate, fail, or otherwise open to break the primary electrical circuit when electrical current through the fuse exceeds a predetermined limit. Upon clearing a fault, the fuse becomes inoperable and must be replaced. Methods and safety practices for determining if the fuse is damaged and for replacing the fuse can be lengthy and complicated.
Another over-current protection device that has conventionally been used is a circuit breaker. A traditional circuit breaker has a low voltage rating, requiring the circuit breaker to be installed in the secondary circuit, rather than the primary circuit, of the transformer. The circuit breaker does not protect against faults in the primary circuit. Rather, a high voltage fuse must be used in addition to the circuit breaker to protect the primary circuit.
Secondary circuit breakers are large. Transformer tanks must increase in size to accommodate the large secondary circuit breakers. As the size of the transformer tank increases, the cost of acquiring and maintaining the transformer increases. For example, a larger transformer requires more space and more tank material. The larger transformer also requires more dielectric fluid to fill the transformer's larger tank.
A load break switch is a switch for opening a circuit when current is flowing. Traditionally, load break switches have been used to selectively open and close the primary and secondary circuits of a transformer. The load break switches do not include fault sensing or fault interrupting functionality. Thus, a high voltage fuse and/or a secondary circuit breaker must be used in addition to the load break switch. The large size of the load break switch and the extra device employed for fault protection require a much larger, and more expensive, transformer tank.
Therefore, a need exists in the art for improved load break switches and over-current protection devices for dielectric fluid-filled transformers. In addition, a need exists in the art for such devices to be cost-effective and user friendly. A further need exists in the art for such devices to be relatively compact.